1. Field of the Invention
This invention relates to a current-controlled oscillator and its use in a phase locked loop system.
2. Description of the Prior Art
Basically a phase locked loop is a frequency feedback system comprised of a phase detector, a low-pass filter and a variable oscillator in the feedback path. When the input signal of the loop is zero the oscillator operates at a predetermined, free-running frequency. If an input signal is applied, the phase detector compares the phase and frequency of the input signal with the signal of the variable oscillator and generates an error voltage that is related to the difference in the two signals. The error voltage is then filtered and applied to the control of the oscillator thereby varying the variable oscillator frequency in a direction that reduces the difference between the two systems. When the frequencies of the two signals become sufficiently close, the feedback nature of the system causes the system to lock with the incoming signal. Once in lock the variable oscillator frequency is identical with the input signal, except for a finite phase difference which is necessary to generate the corrective error voltage to shift the variable oscillator frequency to the input signal frequency, thus keeping the system interlocked.
Data transmitting over a single channel coupled with a pair of transceivers, each of which sends and recieves data, requires that the receiving unit extract not only encoded data but also a timing reference in order to accurately decode the transmitted data. In such a system, it is desirable for one of the transceivers to synchronize its operations to the system providing the data to be transmitted, the other transceiver must then synchronize its operation to the data received from the first transceiver. A basic element required to perform each of the two synchronizing operations is a high-speed oscillator where output frequency can be adjusted by a controlling input.
Normally, a phase locked loop system has a voltage controlled oscillator which runs off the low pass filter output. However, to integrate a voltage controlled oscillator in general requires considerable silicon area on the chip due to the complexity of the circuit. A further problem found in using voltage controlled oscillators is in getting the phase lock loop system to lock on the input signal, i.e., to generate the proper error voltage to regulate the voltage controlled oscillator signal output.
Of general interest in this area is U.S. Pat. No. 4,091,335, which discloses a phase locked loop using a current-controlled ring oscillator. While such a system is effective for the purpose designed, it is to be noted that such system is implemented in bipolar technology, and may lack the flexibility and response needed in certain applications. The current circuit invention is directed to aiding response of the frequency output signal to the current input signal, and includes structure for ensuring that this is achieved, with the circuit being implementable in CMOS technology, to provide all the advantages attendant thereto, including low power draw and high packing density.
Of further interest in this area is U.S. Pat. No. 4,584,695, disclosing as a portion thereof a driver clock generator. However, that circuit portion, involving capacitors, resistors, inverters and amplifiers, does not provide any structure wherein more current may be sinked by current sink means than is sourced by current source means, which would aid response of the frequency output signal to the current input signal.